In the manufacture of high density olefin polymers such as linear ethylene polymers, it is desirable to manufacture such polymers in a range of densities. The density of such polymers can be controlled by copolymerizing varying quantities of a higher monoolefin such as butene or hexene with the ethylene. The density of such copolymers is inversely proportional to the quantity of the higher monoolefin comonomer copolymerized with the ethylene.
It would be desirable to monitor such copolymerization processes by periodically measuring the density of the copolymer being produced and making any indicated changes in the rate of comonomer fed to the reactor to produce copolymer within a preselected range of density. Several problems are encountered, however, which preclude the measurement of copolymer density as a tool to control the density of the copolymer being produced. The first problem is that such polymers as recovered are porous and have a very low bulk density. The samples also have hydrocarbons absorbed in their pores which interferes with accurate determination of density. It has been found that to obtain density values which correlate with polymer properties, it is necessary to extrude the recovered polymer, prepare samples for density determinations, and anneal such samples so that the polymer will crystallize to obtain its maximum density. The preparation of such samples is so time consuming as to render this method inappropriate for use as an online process control parameter. The second problem in using density measurements as a process control parameter is that the presently preferred method for measuring such polymer densities (a density-gradient technique as set forth in ASTM Method D1505- 63T) is laborious and time consuming.
For the reasons discussed above, there is a need in the art for apparatus and methods for rapidly determining the density of olefin polymers, particularly porous olefin polymers having hydrocarbons and/or air absorbed in the pores thereof.